Obama has taken a tough line on Iran, imposing sanctions that have cut the country's oil exports in half. It remains to be seen whether that's enough to prevent the country from building a nuclear weapon.
Romney says he would go even further, taking steps to cut off all of Iran's oil exports. Such a move could send oil prices higher.
What would definitely send oil prices higher is a military strike on the country. It's not thought that either Romney or Obama would order such an attack solely over the nuclear program, but Israel may feel like it has more freedom to act on its own if Romney were president.
